bpf: Show precise rejected function when attaching fexit/fmod_ret to __noreturn functions

With this change, we know the precise rejected function name when
attaching fexit/fmod_ret to __noreturn functions from log.

$ ./fexit
libbpf: prog 'fexit': BPF program load failed: -EINVAL
libbpf: prog 'fexit': -- BEGIN PROG LOAD LOG --
Attaching fexit/fmod_ret to __noreturn function 'do_exit' is rejected.

Suggested-by: Leon Hwang <[email protected]>
Signed-off-by: KaFai Wan <[email protected]>
Acked-by: Yafang Shao <[email protected]>
Acked-by: Yonghong Song <[email protected]>

Author: mannkafai

kernel/bpf/verifier.c

@@ -23932,7 +23932,8 @@ static int check_attach_btf_id(struct bpf_verifier_env *env)
 	} else if ((prog->expected_attach_type == BPF_TRACE_FEXIT ||
 		   prog->expected_attach_type == BPF_MODIFY_RETURN) &&
 		   btf_id_set_contains(&noreturn_deny, btf_id)) {
-		verbose(env, "Attaching fexit/fmod_ret to __noreturn functions is rejected.\n");
+		verbose(env, "Attaching fexit/fmod_ret to __noreturn function '%s' is rejected.\n",
+			tgt_info.tgt_name);
 		return -EINVAL;
 	}
 

tools/testing/selftests/bpf/progs/fexit_noreturns.c

@@ -8,7 +8,7 @@
 char _license[] SEC("license") = "GPL";
 
 SEC("fexit/do_exit")
-__failure __msg("Attaching fexit/fmod_ret to __noreturn functions is rejected.")
+__failure __msg("Attaching fexit/fmod_ret to __noreturn function 'do_exit' is rejected.")
 int BPF_PROG(noreturns)
 {
 	return 0;